By Jonathan M. Hinson
Firefighting foam has been around for many years, from the powder foam to protein foam to the synthetic foam in use today.
Originally designed to extinguish simple hydrocarbon-based fuel fires, today’s foams can be used on Class A fires; alcohol-based fuel fires; and, of course, hydrocarbon fuel fires. Technology has advanced to make foam operations more effective, simplistic, and cost-effective. Flammable and combustible liquids are everywhere and being transported through all modes of transportation. Hopefully, all fire departments have enough foam resources readily available to effect a rescue from a flammable liquid incident. Some communities may have a higher risk or threat, so more foam resources are needed. Whether a department has an eductor and three buckets of foam or thousands of gallons of foam with master stream devices, there are still some basic principles and tips that can apply to both situations to facilitate a successful foam operation.
Whether using onboard foam systems or portable foam eductors, the proper percentage of foam must be educted into the water stream to combat the problem at hand. The proper percentage depends on the fuel burning and, most importantly, the manufacturer’s recommendations. Class B foam is used at one, three, or six percent, describing the percentage of foam concentrate in the foam solution. The manufacturer through testing has determined at what percentage foam should be used. If the foam is alcohol-resistive, then the recommendations will also cover what percentage is appropriate for both hydrocarbon-based fires and polar solvents.
These recommendations are generally clearly stated in big numbers on the foam’s storage container. Using a percentage less than the recommendation will make the operation ineffective; increasing the percentage can make operations more effective with quicker extinguishment. However, when using a higher percentage, foam system operators must understand that they will use double if not triple the amount of concentrate required, potentially depleting the concentrate supply prematurely.
|1 Photos by author.|
Class A foams are generally used at 0.1 to 1.0 percent, once again depending on the manufacturer’s recommendations. In most cases, the lower percentages are used for fire attack and extinguishment while the higher end of the range can be used for exposure protection. Using higher than recommended percentages of Class A foam generally does not result in more effective operations like seen with Class B foams.
Use onboard foam systems and eductors to properly mix foam concentrate and water to make foam solution. You must perform this process flawlessly to have a successful foam operation. However, the foam concentrate must be made into finished foam through proper application to have a completely successful foam operation. The foam must be applied continuously and without failures. Proper application is critical with Class B foams, as they make a film or membrane to properly extinguish. Class A is a direct application just like water, so no film or membrane is produced.
The ensuing discussion about foam application applies to Class B foams. In basic training, firefighters are taught to apply foam with three different methods: roll on, bank on, and rain down. Nozzle operators must ensure that the stream is not plunged into the product, causing the foam to be pushed under the product and thus less effective. If the fuel is alcohol-based, the foam will completely break down if plunged because the water in the foam will quickly mix with the alcohol, rendering the foam useless.
When using a straight stream on dirt, the stream will start to drill a hole in the ground. This is the plunging that cannot occur on a flammable liquid fire, especially an alcohol-based fire. The stream needs to land gently on the surface like snow. When using the rain down method, use a wide gentle pattern rather than a straight stream to allow for an easy landing without much plunging under the surface of the fuel. Properly designed (Class B) foam nozzles have a pattern that will allow the foam to land on the surface of the fuel much more gently like snow rather than plunging under the fuel surface.
Selecting the right nozzle for the job is key. The nozzle adds the final two aspects to finished foam: air and agitation. The amount of air and agitation determines the expansion ratio achieved. Different expansion ratios have different applications; it all comes back to the size of the bubble in the foam.
The lower the level of expansion, the less air is entrained in the finished foam, which produces much smaller bubbles. The smaller bubbles in lower-expansion applications have better and quicker fire knockdown, making them good for combating flammable liquid fires. The smaller bubbles are much more fluid, moving across the surface of a burning fuel much more quickly as compared with the larger bubbles produced in a medium-expansion system. The disadvantage of the low-expansion operation is that the water is going to drain out of the foam blanket much faster, breaking down the foam faster and allowing the flammable vapors to escape and potentially find another ignition source.
As expansion ratios increase, the size of the foam bubble increases, which allows the foam blanket to stay intact longer with slower water runoff. This lessens the need to reapply foam while waiting for cleanup to be completed. The less times firefighters have to reapply foam, the less concentrate is used and the less runoff that has to be collected and disposed of. With these bigger bubbles, the finished foam is not as fluid and will not quickly and easily spread across the product’s surface as the smaller bubbles do. To be effective and achieve complete coverage, you will have to apply medium expansion from multiple directions and locations.
Once you have determined an expansion ratio, you can select the correct nozzle for the job. For Class B foam, a smooth bore nozzle is not effective and is not recommended for use. With bubbles not being important with Class A foams, a smooth bore or any nozzle can be effectively used to apply Class A foam. With Class B foams, a standard fog nozzle can be used and will provide a 2:1 to 4:1 expansion ratio. When using a standard fog nozzle, the aeration occurs when the small water droplets are pushed through the air. The agitation occurs when the water droplets run into each other in the stream and when the stream hits the object, which means the bank on and roll on application methods are the best choices with a standard fog nozzle.
A specially designed foam nozzle will provide expansion levels of up to 20:1 when used properly. These nozzles have a collar or other means by which air is pulled into the stream before it leaves the nozzle and have rods or spikes that the stream is pushed through to agitate the stream. Most of these have a tube that the stream passes through with the air being pulled in at the base of the tube and spikes or rods in the tube to agitate the stream. Photo 1 (left) shows a nozzle that is specially designed for foam operations. Many manufacturers make attachments for standard fog nozzles to make them foam nozzles. Photo 1 (right) shows a standard fog nozzle with an attachment on the tip for foam operations. With the stream passing through all the agitation, a foam nozzle does not have the reach of a fog nozzle because the energy and velocity are lost.
A standard fog nozzle may be the best choice when a longer reach of the stream is needed because the nozzle has nothing to agitate the stream. The more the stream is agitated, the more energy is removed from the stream, causing it to be propelled less once it leaves the nozzle. A fog nozzle has no additional agitation, so the full reach of the stream can be achieved. A specialty foam nozzle has devices (rods and spikes) the stream passes though that slow down the stream, thus reducing reach.
Should a higher level of expansion be needed such as for vapor suppression on an unignited flammable liquid spill, a medium-expansion nozzle is best. This style of nozzle will provide an expansion ratio greater than 20:1, which is a shaving cream-type consistency. Having the higher expansion ratio will cause the foam blanket to last longer or not break down as fast, preventing the need to reapply as often and reducing the amount of water and concentrate needed. The tubes on the nozzle are much wider than a standard foam nozzle and have multiple levels of screens inside to aerate and agitate the foam solution. This nozzle is generally not used on ignited incidents because it has no reach – the foam solution just dribbles out of the nozzle. Photo 2 shows a medium-expansion nozzle in use. Note the very thick foam blanket produced and the limited reach of the stream.
It is critically important when using any type of nozzle with an adjustable flow to ensure the flow selected on the nozzle matches the flow of the eductor. Otherwise, this can create back pressure, rendering the operation ineffective. The eductor controls the flow in a foam operation, not the nozzle.
When applying foam with any type of nozzle, the nozzle operator must begin flowing away from the intended target until foam reaches the nozzle and a proper attack can be made with foam. Immediately opening the nozzle onto the target will just add water and allow the fuel to spread farther. The time it takes from when the nozzle is opened to when properly proportioned foam solution arrives is known as transit time. It takes time for the venturi to get foam up into the hose and time for the foam to travel down the hoseline to the nozzle. In a 200-foot 1¾-inch hoseline, transit time can be up to 30 seconds. This will seem like an eternity when you are looking at a pool of burning flammable liquid; however, operators must let this time pass before shutting down and starting to look for another problem. Any time you shut down the nozzle, the transit time will start all over again, further complicating the perceived problem.
Applying foam for the correct amount of time is another factor that is critical to foam application success. National Fire Protection Association (NFPA) 11, Standard for Low-, Medium-, and High-Expansion Foam (2016 ed.), provides guidance on how long foam should be flowed onto different flammable and combustible liquid incidents. For any type of spill fire (product less than one inch in depth), flow foam for a minimum of 15 minutes. For a fire in depth (such as a tank), the flow time depends on the flash point of the material burning. For hydrocarbons with a flash point below 100°F and all crude petroleum (no matter what flash point), the foam must flow for a minimum of 65 minutes, and anything above 100°F must flow for at least 50 minutes. Any fire in depth involving an alcohol-based fuel must maintain a foam flow for at least 65 minutes unless the manufacturer states otherwise. These numbers are based on using handlines and master stream devices; using fixed foam systems and other considerations can change the required time. Consult the standard for more in-depth information.
NFPA 11 also provides guidance on how many gallons per minute (gpm) of foam solution are needed for the time prescribed. Just like structural firefighting, not having enough gpm will result in an unsuccessful operation. For fires in depth, 0.16 gpm are required for every square foot of surface area for hydrocarbon-based fuels. For polar solvent fuel fire in depth, consult the manufacturers’ recommendations for flow rates, but they generally are between 0.16 and 0.30 gpm per square foot. On hydrocarbon spill fires, 0.16 gpm per square foot are required when using older protein foams and only 0.10 gpm per square foot are required when using aqueous film-forming foam (AFFF) or alcohol-resistant (AR)-AFFF. For alcohol-based spill fires, consult the manufacturers’ recommendations according to NFPA 11. When calculations produce a flow that is not possible with the devices on hand, be sure to round up to the nearest available flow. Flowing less than the required amounts will cause the operation to fail. NFPA 11 has more comprehensive information and additional considerations concerning needed foam flows.
When determining the amount of foam flow needed for an operation, there is another important consideration: the amount of waste or amount of foam that does not reach the target. NFPA 11 required flows are what you need to reach the target, not the amount that needs to be discharged from a nozzle. Wind, poor aim, and other factors can cause a portion of the stream to be lost before reaching the target. When calculating the needed flow, add 25 percent to the final number to account for waste. Should extremely high winds or other adverse conditions be present, the percentage of waste should be increased.
Using the required flow time and required flow based on square feet (plus waste) provides vital information for a successful foam operation. Calculations using this information will provide the total gallons of concentrate needed for the incident as well as the required water flow. You can use slide charts from foam manufacturers for simpler calculations or develop your own spreadsheets for preplanning and quick on-the-fly calculations. Photo 3 shows an example of a slide chart made by a foam manufacturer to assist firefighters with determining foam needed for different size fires. If there is not enough concentrate or water available, do not start the operation until those resources are on scene and available. For water supply, this may require finding additional hydrants, adding tankers to a water shuttle, or using a combination of a water shuttle and hydrants. When figuring water supply, do not forget the water needed to cool any exposures in the area. You can request additional foam concentrate from neighboring departments, industrial facilities, and even emergency shipments from manufacturers. An accepted practice is to have double the amount of foam concentrate needed on scene to allow for problems and postfire security.
Even after you have applied the foam and extinguished the fire, there is one more way the foam operation can fail. Walking through a foam blanket breaks the film or membrane that is preventing the production of the burnable vapors. The film from an AFFF will repair itself, but there is still the opportunity for vapors to escape and ignite, causing injury to the person walking through. The membrane of an AR-AFFF foam will not repair itself, creating a lasting problem. If someone must walk through a foam blanket made from AR-AFFF (for hazmat monitoring or to close a valve, for example), foam will need to be flowed onto those walking through the foam to protect them and ensure no holes are left in the foam blanket.
In a basic foam operation, there are not many moving or mechanical parts, but the operation can still be rather complicated and confusing. The same holds true of larger complex operations. Everything must come together exactly as designed, or the foam operation will fail. Take the time to plan the desired foam operation and ensure every part meets the requirements. When the operation starts and the desired results are not obtained, stop and review the operation step by step to find the problem. It may not always be obvious, so double and triple check and get others to check behind you. Do not let something simple ruin a critical foam operation.
To have a successful foam operation, you must use the correct foam (which has been properly stored), use the equipment as designed and maintained, and apply the foam correctly. This article provides some tips on preventing problems and failures. Departments must get out and train on their foam equipment to ensure they understand the operation and to ensure there are no problems with the equipment. With continual studying and practice, you can prevent foam operation failures.
JONATHAN M. HINSON is a captain and a 15-year veteran of the Chesapeake (VA) Fire Department, serving on the department’s Foam Team. He is a 20-year fire service veteran. He also serves as chief of the Newsoms (VA) Volunteer Fire Department.